Oilpan: Remove ~HTMLMediaElement document access when the document is destructed.

The document() accessor on all Nodes end up touching the document. The
reason is that the document accessor gets the document from the
TreeScope which is often the document itself. Therefore, we cannot
use the document() accessor in Node destructors. Fortunately, there
is a more direct way of checking if the Document has been destructed.
HTMLMediaElement is an ActiveDOMObject which is a
ContextLifecycleObserver. Therefore, ActiveDOMObject::executionContext
will tell us if the Document is still alive so that we can access it.

R=haraken@chromium.org, oilpan-reviews@chromium.org, sigbjornf@chromium.org, zerny@chromium.org

Review URL: https://codereview.chromium.org/310333003

git-svn-id: svn://svn.chromium.org/blink/trunk@175465 bbb929c8-8fbe-4397-9dbb-9b2b20218538